1. Field of the Invention
The present invention relates to an intelligent system and method for monitoring a generator reactive power limit using machine model parameters, and more particularly, to an intelligent system and method for monitoring a generator reactive power limit using machine model parameters, which can calculate a maximum reactive power limit corresponding to over-excitation and a generator terminal voltage corresponding to under-excitation, estimate a correct field current even when system variable are changed, and monitor the generator reactive power limit by using machine model parameters and a one-machine infinite bus, to thereby supply a maximum or minimum reactive power to a power system within allowable generator reactive power limit and prevent a generator trip caused by the reactive power limit and a power failure over a wide area.
2. Background of the Related Art
References for the present invention are as follows:    [1] CWG & MVWG, “Test Guidelines for Synchronous unit Dynamic Testing and Model Validation”, February, 1997, WSCC. www.wecc.biz;    [2] P. Kundur, Power System Stability and Control, PP. 101-102, 1994, McGraw-Hill;    [3] F. P. de Mello, L. N. Hannett, “Representation of Saturation in Synchronous Machines”, IEEE Transactions on Power Systems, Vol. PWRS-1, No. 4, November 1988, pp. 8-18;    [4] F. P. de Mello, J. R. Ribeiro, “Derivation of Synchronous Machine Parameters from Tests,”, IEEE PWR Apparatus and Systems, Vol. PAS-96, No. 4, July/August 1977;    [5] EPRI Report EL-1424, “Determination of Synchronous Machine Stability Constants,” Vol. 2, prepared by Ontario Hydro, December 1980;    [6] EPRI Report EL-1424, “Determination of Synchronous Machine Stability Constants,” Vol. 3, prepared by PTI, December 1980;    [7] Bharat Bhargava, “Synchronized Phasor Measurement System Project at Southern California Edison Co.”, IEEE PES SM 1999, pp. 18-22, 1999;    [8] Magnus Akke, “Phasor Measurement Applications in Scandinavia,” IEEE PES T&D Conference and Exhibition 2002: Asia Pacific, pp. 480-484, 2002;    [9] Report, “Aug. 14, 2003 Outage Sequence of Events”, U.S./Canada Power Outage Task Force, Sep. 12, 2003;    [10] G. W. Stagg and A. H. Abiad, Computer Method in Power System Analysis McGraw-Hill, 1968;    [11] C. Lemaitre, J. P. Paul, J. M. Tesseron, Y. Harmand, and Y. S. Zhao, “An indicator of the Risk of voltage Profile Instability for Real-Time Control Applications,” IEEE Summer Meeting 1989, Paper 89Sm713-9 PWRS;    [12] V. Ajjarapu and C. Christy, “The Continuation Power Flow: A Tool for Steady State Voltage Stability Analysis,” IEEE PICA Conference Prodeedings, pp. 304-311, May 1991;    [13] N. Flatabo, R. Ognedal, and T. Carlsen, “Voltage Stability Condition in a Power Transmission System Calculated by Sensitivity Methods,” IEEE Trans.;    [14] C. Lemaitre, J. P. Paul, J. M. Tesseron, Y. Harmand, and Y. S. Zhao, “An indicator of the Risk of voltage Profile Instability for Real-Time Control Applications,” IEEE Summer Meeting 1989, Paper 89Sm713-9 PWRS; and    [15] TEST GUIDELINES FOR SYNCHRONOUS UNIT DYNAMIC TESTING AND MODEL VALIDATION, 1997, WSCC.
A generator reactive power limit is related to voltage stability of a power system. The voltage stability is detected using a method of monitoring a bus voltage of the power system (refer to references [11], [12], [13] and [14]). A large-scale power failure due to an over-excitation trip of a power plant has recently occurred (refer to reference [15]), and there is every possibility that a power failure occurs because of a trip caused by over-excitation or under-excitation of a power plant. Accordingly, a method of effectively monitoring the generator reactive power limit is required. However, conventional techniques cannot meet this requirement.
A conventional method of monitoring a generator reactive power limit analyzes and judges the current generator operating state by a field generator operator using a capability curve, an under-excitation limiter (UEL) limit, and an over-excitation limiter (OEL) limit provided by a generator manufacturer. This method is an approximate technique depending on the capability of the generator operator, and thus it is difficult to estimate a correct generator reactive power operation when power system variables are changed. That is, the conventional generator reactive power limit monitoring method has the following problems.
Firstly, it is impossible to calculate a reactive power limit with respect to an OEL generator field current limit and indicate the calculated result on a generator reactive power capability curve. In general, a method of monitoring the reactive power limit of a generator while the generator is operating uses a generator reactive capability curve illustrated in FIG. 1. The generator reactive capability curve illustrated in FIG. 1 indicates an allowable reactive power at 500 MW generator power under 60 PSIG hydrogen pressure. A minimum reactive power limit according to under-excitation can be monitored by indicating an UEL limit on the generator reactive capability curve as illustrated in FIG. 1. In terms of monitoring of a maximum reactive power limit according to over-excitation, however, an OEL limit is set to a generator field current ifd, and thus it is difficult to calculate and indicate a reactive power limit with respect to the OEL generator field current limit. This is because the maximum reactive power limit is varied according to a generator output condition, a terminal voltage and a system voltage. Accordingly, a method of indicating the reactive power limit with respect to the OEL generator field current limit on the capability curve is required.
Secondly, it is impossible to calculate or estimate a terminal voltage corresponding to a reactive power limit with respect to an UEL and indicate the terminal voltage on the capability curve. An UEL limit corresponds to a ratio of a reactive power Qe to an active power Pe, and thus the UEL limit can be indicated on the capability curve as illustrated in FIG. 1. However, the terminal voltage of a generator is important in the actual operation of the generator. The conventional technique has difficulties in calculating or estimating the terminal voltage corresponding to the reactive power limit with respect to the UEL and indicating the terminal voltage on the capability curve.
Thirdly, it is impossible to estimate a variation in power system variables and indicate a maximum generator reactive power limit on the capability curve. A generator is not operated at a single operating point for ceaselessly generating large and small power system disturbances. For example, a generator terminal voltage, a generator active power, a generator reactive power and a network voltage change momentarily. If a generator operator can estimate these power system variables and prepare for a variation in the power system variables, power system reliability is improved. However, there is a limit in this manual method, and thus a method of estimating a variation in the power system variables according to a variation in generator power and automatically indicating the maximum generator reactive power limit on the capability curve is required.
Fourthly, a reactive power limit with respect to a generator over-voltage limit and a generator under-voltage limit cannot be calculated. The generator reactive power limit is affected by the generator over-voltage limit and the under-voltage limit as well as the OEL limit and the UEL limit. Generally, a generator over-voltage and a generator under-voltage are respectively 105% and 95% of a rated voltage. Accordingly, a method of calculating the reactive power limit with respect to the set generator over-voltage limit and the generator under-voltage limit is needed.